The typical arrangement and operation of computer networks employing mobile-IP communications is well known. As a general matter a mobile node obtains a mobile-IP address (“home address” or “HoA”) from a mobile IP home agent and registers with the home agent to let the home agent know the network where the mobile node is currently operating. When IP communications are sent to the mobile node's home address, the communications are conventionally routed to the home agent. And based on the mobile node's registration, the home agent then routes the communications to the mobile node's current serving network. Upon receipt of those communications, the mobile node's serving network then forwards the communications locally to the mobile node.
More specifically, each serving network in a mobile-IP system may include a mobile IP foreign agent (e.g., router, network access server, or the like) having an IP address referred to as a care-of address or “CoA”, and the foreign agent broadcasts its care-of address in a “router advertisement message” (“ICMP Type 9” message) for receipt by mobile nodes operating on the foreign agent's network. When a mobile node receives a router advertisement message that carries a new care-of address (one with which the mobile node is not currently associated), the mobile node then assumes that it is operating on a new serving network. Accordingly, in response, the mobile node sends a mobile-IP registration request (RRQ) message to the mobile node's home agent (via the foreign agent), to register the new care-of address in connection with the mobile node's home address. In turn, the home agent then returns a mobile-IP registration reply (RRP) message to the mobile node (via the foreign agent), designating the home address to be used by the mobile station.
Through this process, a communication tunnel is effectively set up between the mobile node's home agent and the mobile node's foreign agent, since the home agent is set to forward to the foreign agent all communications that are destined to the mobile node's home address, for local transmission in turn from the foreign agent to the mobile node. Thus, in practice, the mobile node may include its home address as the source address in any outbound packet communications that it sends. Packet communications that are then sent in response to the mobile station's home address would then be routed to the home agent, since the home address is associated with the home agent. In turn, the home agent would then pass those communications to the mobile node's care-of address at the foreign agent, and the foreign agent would then pass the communications locally to the mobile station.
In general, when a home agent receives a mobile-IP RRQ that designates a new care-of address for a mobile node, the home agent will tear down the tunnel that the home agent has with the mobile node's old care-of address, i.e., with the mobile node's old foreign agent. In particular, the home agent will update its registration record for the mobile node to correlate the mobile node's home address with the new foreign agent's care-of address. That way, when the home agent subsequently receives communications destined to the mobile node's home address, the home agent will forward them to the new foreign agent rather than the old foreign agent, for transmission in turn to the mobile node.
In many networks, the foreign agent may further serve to track communications by mobile nodes, for billing and other purposes. In particular, communications from the mobile node may pass through the foreign agent on their way to their destination, and, due to the very nature of mobile-IP, communications destined to the mobile node's mobile-IP address will also pass through the foreign agent on their way to the mobile node. In practice, as these communications pass through the foreign agent, the foreign agent may log the communications, such as by reporting communication start and stop times and/or data quantities to an authentication, authorization, and accounting (AAA) server that works with a billing system. Accordingly, the mobile node user can be billed for sending and receiving packet communications.
In some mobile-IP implementations, each serving network will have a mobile-IP foreign agent, and a centralized home agent will be provided separately from the foreign agents (e.g., on a central network connected with the serving networks, or on one of the serving networks). This implementation is commonplace in cellular wireless networks for instance, where each serving network has a packet data serving node (PDSN) that serves as a mobile-IP foreign agent for mobile nodes operating in the serving network, and at least one centralized home agent is provided separate from the PDSNs, typically on a wireless carrier's core packet network. In such an arrangement, a mobile node is always located on a “foreign network,” even if the mobile node is located on its home network as well, since the PDSN currently serving the mobile node operates as a foreign agent for the mobile node.
In other mobile-IP implementations, a mobile node may have a home serving network, and the home network may include a server that functions as both a home agent and foreign agent for the mobile node, and as a foreign agent for visiting nodes. In such an implementation, the home network server may assign a mobile-IP address to the mobile node, just as a home agent would normally do. However, no tunnel would need to be set up for the mobile node when the mobile node is operating in its home serving network, since the home agent is the foreign agent.
In such an implementation, when the mobile node roams to another serving network and detects a router advertisement message that designates the care-of address of the foreign agent on the new serving network, the mobile node may then register with the home agent in the mobile node's home network, to register the new care-of address in connection with the mobile node's mobile-IP address. That way, communications destined for the mobile node's mobile-IP address will be routed to the mobile node's home agent in its home network. Conventionally, the home agent will then forward the communications to the mobile node's registered care-of address, which will be the address of the foreign agent in the new serving network, and the foreign agent will pass those communications locally to the mobile node.
In turn, if the mobile roams back into its home network, the mobile node would similarly receive a router advertisement message from the foreign agent on its home network. However, in this case, the router advertisement message would carry as a care-of address an IP address that is the same as the IP address of the mobile node's home agent, since the foreign agent is the home agent. Conventionally, the mobile node will then again transmit a mobile-IP RRQ message to its home agent, providing the care-of address received in the router advertisement message. In this case, however, since the care-of address in the RRQ message would match the home agent's own IP address, the home agent would responsively tear down the tunnel that it had with the foreign agent on the mobile node's last serving network. At that point, any communications destined to the mobile node's mobile-IP address will be routed conventionally to the mobile node's home agent, and the home agent (functioning as foreign agent) will forward those communications locally to the mobile node.
A great advantage of mobile-IP communications is that a mobile node can seamlessly move from serving network to serving network, and communications to the mobile node's mobile-IP address will in theory still make it to the mobile node notwithstanding that movement. In particular, each time the mobile node moves to a new serving network, the mobile node may simply re-register with the home agent, so that the home agent will in theory always have a record of the mobile node's current care-of address and thus where to send packet communications that are destined to the mobile node.